1. Field of the Invention
The invention relates to the isolation of pressure sensing devices from environmental contaminants, and more specifically, to achieving optimal coupling between a diaphragm exposed to a measurand environment and a pressure sensing device isolated within a protective housing.
2. Description of the Related Art
Pressure sensors which include a protective thermoplastic housing with a pressure sensing device isolated inside are well known. Moreover, the use of a diaphragm secured to the housing in conjunction with a substantially non-compressible pressure transfer medium within the housing to couple the diaphragm to the pressure sensing device is well know. Unfortunately, however, there have been problems with the use of such diaphragms in such pressure sensors.
In particular, U.S. Pat. No. 4,686,764, entitle "MEMBRANE PROTECTED PRESSURE SENSOR" which issued to Adams et al., on Aug. 18, 1987 discloses the sensor in FIG. 1 which includes a diaphragm 32 which is rolled into place as shown by the arrow 33. A problem associated with the approach suggested by Adams et al., is that it does not provide reliable initial pressurization sensitivity. Another problem is that sensitivity may vary over time. Furthermore, the proposed method of manufacture can be time consuming, costly and somewhat unreliable.
U.S. Pat. No. 4,993,265 entitled "PROTECTED PRESSURE SENSOR AND METHOD OF MAKING" issued to Koen et al., on Feb. 19, 1991 discloses the poured in place diaphragm shown in FIG. 2. While this diaphragm generally has been successful, there have been shortcomings with its use. For example, the surface of the diaphragm 32 shown in FIG. 2 has a relatively high co-efficient of friction, and therefore, can tear relatively easily. Moreover, the diaphragm 32 is secured to the header 11 using an intermediate primer which unfortunately can result in a somewhat unreliable (and process dependent) bond. Moreover, the diaphragm 32 has been found to be relatively permeable to environmental gases. Additionally, the sensing element 14 is mounted so as to receive a top-side pressure which can result in incremental damage to the wire bonds 18 over time.
U.S. Pat. No. 4,920,972 entitled "GEL-FILLED BLOOD PRESSURE TRANSDUCER" which issued to Frank et al., on May 1, 1990, includes the diaphragm 30 shown in FIG. 3. That diaphragm 30 has a rim 31 which is inserted into an annular channel 29 in order to mechanically hold the diaphragm 30 in place. A difficulty with the approach taught by Frank et al. is that the diaphragm 30 must be biased slightly in order to distend the diaphragm, presumably to remove any irregularities in the diaphragm surface which may have been introduced when the diaphragm was mechanically assembled.
Yet another earlier pressure sensor 105 is shown in FIG. 4. The sensor 105 includes a housing 106 and a gel-filled T0-8 sensor package 108 mounted on a circuit board 110. A polyurethane diaphragm 112 is attached to the top of the housing 106 using an acrylic adhesive. The housing is filled with a non-compressable pressure transfer medium such as a gel. The T0-8 package 108 is inserted into the gel within the housing so that the gel within the T0-8 serves as a pressure transfer medium which couples pressure from the diaphragm 112 to a pressure sensing device (not shown) within the housing. One problem experienced with pressure sensors of the type illustrated in FIG. 4 has been failure of the adhesive due to corrosive solvents in the environment.
Thus, there has been a need for an improved pressure sensor. The present invention meets that need.